System for preventing electrical ignition of reservoir stored inflammables



July 4, 1933. E. R. SCHAEFFER 1,916,335

SYSTEM FOR PREVENTING ELECTRICAL IGNITION OF RESERVOIR STOREDINFLAMMABLES Eiled Jan. 17, 1927 a Sheets-Sheet 1 July 4, 1933. E, R.SCHAEFFER 1,916,335 SYSTEM FOR PREVENTING ELECTRICAL IGNITION OFRESERVOIR STORED INFLAMMABLES 1927 3 Sheets-Sheet 2 Filed Jan. 17

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C ix-nu July 4, 1933- E. R. SCHAEFFER SYSTEM FOR PREVENTING ELECTRICALIGNITION OF RESERVOIR STORED INFLAMMABLES Filed Jan. 17, 1927 3Sheets-Sheet 5 Patented July 4, 1933 ELMER n. sonanrrnn, or cansrwoonNEW von asslenon, BY ass e ASSIGN- MENTS, T JOHNS-MANVILLE eonroserion,or new YORK, N. Y., aoonrogen'r on OF NEW YORK SYSTEMFORPREVENTINGELECTRICAL IGNIT'JZONOF nnsnnvotn smonnn- INFLAMMAB'LESI ev V s ites-rimmed Janriaryll 1927. Serial no. 151,623..."

storage'structuresor containers which may be pools or reservoirs partlyexcavated or surrounded by earth embankmentse These of the order oi. 500to 600 feet in diameter. if eircular and of greater major. length itoval. Naturallytl esereservoirs are placed Where thesinitaoe'contourenables the retaining embankment i191: be constructed of earthexcavated; for the poolior basiniproper. Pre

vention of; evaporation losses dictates a root or cover of someikind forthese great'pools.

of oil.

Experience has shown that thQSQ IGSGEVOUS arealmostasvulnerable toriskfrom fire by electi.caleignition the above groundnme tallic-Wallfo ltanks. which are notoriously prone to fire from this cause.

One ohjectof thisinvention isto provide a system of protection adaptedto minimize the risk o't-occurronce of an inflaming electrical spark .inthe vapor-spaces above the:

oil and beneath the root or coverv otthe steragerontainer or aroundtheeaves off-the root or cover.

It Will be understood that 'the :risks from atmospheric electricalphenomena are not confinedto actual lightning strokeson 'thereservoirsor their roots. Most. of the fires which havedestroye-d greatquantities ot oil stored in these reserroirs have occurredduring-lightningstorm conditions, it is true,biit in nearly all casesthe actual ignition coulc ri dtbe traced to lightning bolts 'strihingator near the pool. or reservoir destroyed.

Li htningdischar es astar a'wa as a mileor L" C L several hundred yardshave generally been storage structures are sometimes verydarge,

are included in the classot. phenomenameferred to.

In the accompanying.drawings, .the -invention isshown by ay-of exampleas ap- PlIQCl: to' the protection of anoilstoragecontainer of the earth,reservoir type as -.illustra.-; tive of some of the torms in which thein-L vention may-,be expressed.

Fig. 1 -i s.a,.plan view of atlarge reservoir and the surroundingterritory, serving-inathe nature ofan explanatorydiagram of theef-z feetof andreasongtor certain preventive elec-- ticaldevices';

Fig;v l isa corresponding detailelev-atiom. Fig. 2-is a similar planillustrating amid-- y as" tiple group of three reservoirs and an arrangement of protective devices iforthese res-Q ervolrs;

Fig; 8 is a. fragmentary enlarged plan. of. an ovalireservoir showing ona largerscale arts of the protective devices;

Fig,- 4: is :a; corresponding vertical section on a still. larger scale,for

line l -dot Eig. 3 and Fig. A is an enlarged detail of devices 7:examp1e. on the slopes 2-and bottom 3 having amasonrylin ing,- usuallyof reinforced concrete, serving as an-oil-proot container to hold thepoolot oil. The'reintorcement maybe for example a 6 in. X 6 in. Wire orbar'vvelded mesh 4: laidf continuously through the sheet masonryfi and3; for strengtlnmigidity and economy of materialsthe Wide sheets ofvconcrete maybe reinforced atintervals asby integral ribs at 5;. theremaybe series of these orossing-geachother for example atright angles.The in:

tersect onsat 5 may be provided. w th ele- -vated bases.6 6wfor a systemofivertic-alstruts,

7 adaptedto support the usually Wooden rafters 8 of a rooff). TheI'OOf9-llS112LllYCOHl-g.

prisssiplankingtcovered' Withitin. or saturated 5 Water gutter trough at11 for a more or less Qcirese'rvoirs or tanks, and in the present aspecteffective Vapor seal.

Referring noW to Fig. 2, such reservoirs are naturally and necessarilythe terminal points of considerable systems of metallic piping. Thesepipe systems may carry Water for drainage from the roof structure; theymay be provided for fire extinguishing purposes; they may comprise theinlet and outlet pipes for filling and emptying the reservoirs. Such apiping system for reservoirs R R R is indicated generally at a in Fig.2. For the purposes of the present description, What these pipes serveto carry is of no importance; they are an inevitable accompaniment ofthe they constitute a system of electrical 'conductors in haphazarddirections terminating at the pools or reservoirs and, so far as theythere find and are connected purposely or Cinc'identally to conductingstructures in or about the metallic reinforcements of the concrete, theWater gutters at 11 or the roof at 9 of the typical structure shown inFig. 4, they are conductors ending in a large area having6:;zhighelectro'static capacity. It is true that in thoroughly WetWeather the pipe lines leading to such a tank or reservoir arerelatively Well-grounded throughout their length to earth, and there isthen no preferential area qfor the development of high-potentialelectric charges, for example 011 the roof of a reservoir at-the end ofa pipe system 1, but itis acommonplace that tanks or reservoirs of thekind here in question are in most in q qstances in arid or semi-aridregions, Where the moisture content of the ground is always relatively10W andsometimes practically nonexistent. When the earth is dry, it willbe observed that the reservoirs are termini of electrical conductivesystems along the pipes leading to them, and are almost ideally adaptedto receive the high terminal potentials of charge resulting fromelectrical surges along the pines. Whether or not the reservoir roof ,isthe terminus of a surge, difference of potential from one part toanother part of the roof or between the roof and the surrounding earthmay readily develop from other causes operative Whether the earth is dryor Wet.

Referring for example to Fig; 1, let us suppose that thunder-stormconditions prevail. In that case a highly charged cloud may pass abovethe country shown in diagram in Fig. 1, and that point of this cloudWhich is nearest the earth in an electrical sense, and therefore adaptedto become the taking-off spot of a lightning bolt, Will travel inrespect to the surface illustrated in the figure and may occupy anypoint on it. Sup- I pose, for example, this discharge point of theoverhead cloud is over the point as on Fig. 1. A coordinate result ofthis condition Will be thatthe equivalent and opposite electricalpotential of the earthsurface will increase at point an, about to becomeor possibly to become the terminus of the electrical discharge fromearth to cloud, or cloud to earth. Since it is clear that the point ca-is not at rest so far as motion in the plane of the earths surface isconcerned, in the instance given the point 00 Will be any place ofhighest potential of the earth in respect to the cloud, and thispotential Will be attained and maintained to lightning stroke rupture byan electrical flux relative to the point a; to or from all directions onthe earths surface. The surface fluxes on the earth are of coursecomplicated by proper motion of the point :0 as the cloud moves. Iftheconductivity of the earths surface may be assumed to be uniform exceptas it is interrupted by the structures in and about the reservoir R,'itmay be observed that Whatever the position of the point as or Whateverits Wanderings, there Will be electrical flux across the structures inand about the reservoir R and therefore there will be lateral differenceof potential due to the flow toward or away from the point 0:. Assumenow that there is disruptive discharge of a lightning stroke to and atthe point an; equilibrium of the surface charge of the earth ensues forthe moment, but only after an opposite flux across the reservoir R, theflow radiating away from the point a3.

The flux on the surface of the earth during a thunder storm is far froma negligible factor in the case of such large structures as thereservoirs above mentioned, and I attribute to such fluxes thedestruction of 'several such reservoirs. At the entirely reasonablefigures of 100,000,000 volts for the potential from cloud to earth and amaximum current on lightning discharge of 100,000 amperes, the flowinterrupted by the tank protecting structures as shown in Fig. 1 for adischarge relatively to the point x is amply suflicient to account forserious sparking between metal or Wet parts of the roof, about the eavesof the roof, between elements of the concrete reinforcement, andespecially between any pipe connections or conductor connections topoints near the roof of the tank at distances of the point x measureablein miles- This invention provides for deviating the potentialdifferences which might result in sparking in or about the roof, roofsupports, eaves, from one part of the roof to another part of the roof,or from one part of the reinforcements or Water layer on the roof or theretaining embankment to another part, on the occasion of either aninduced surge in the conductor leading to the reservoir or on theoccasion of a lightning stroke. The

newest:

structures mainly. depended upon for these provisions are recommendedtocomprise a conductor'system' adapted at the same time to provide a shuntpath" for surface flows 3 on the surrounding surface of theearth 6X- 1the roof of the reservoir, may in some cases be wholly relied upon,butin view of the large quantities of surges adapted to flow in thenecessary metallic or other conductor systems terminating in the tankroof and for other reasons it is preferred to supplement this ringconductor by an overhead network or grid. There is'thus secured theadvantage of elevating away from the tank roof the equipotentia-lsurface correspondingto the surface of the earth, so that in anelectrical sense the reservoir roof and the oil inthe rcservoir'Jmay-beregarded as having been placed underground, provision also having beenmade of an artificial low resistance shunt for earth-surface fluxesquite harmless in respect to producing sparks invapor bearing spaces inand near the reservoir.

I have jointly with Leslie A. Baldwin in my application for LettersPatent, No. 32,738,

'2 area May 1925, Patent No. 1,617,788,dated February 15, 1927,described and claimed a system of spaced earthed conductors spaced awayfrom the uppersurface of an insulating tank roof and" concentratedbycloser 'i spacing at the regions of-maximum potential gradient betweenthe tank top and an elevated charged atmospheric stratum, which systemhas been found effective forthe protection of the tank from directlightning strokes and for the harmless discharge of accumulated chargesof atmospheric electricity; a species of this device is perfectlysuitable for the purposes abovementioned;

For example, as best shown in Figs. 3 and 4, the entire reservoir may besurrounded by a ring conductor thoroughly grounded as at 26, 26 to anyentering pipes, and: expediently elsewhere earthed, by driven metalposts, for example, everywhere about the reservoir as indicated at 27,preferably connected-to the concrete reinforcement 4 (units of which maybe'bonded together electrically) as at 29; and this conductor 25'may besystematically connected by radial conductors 28 to anoverhead grid ornetwork 30 suspended at a substantially uniform dis tance everywhereabove theroof 9 of the reservoir. Preferably the ring conductor25 issupplemented by an elevated marginal con- 'ductor extending around thereservoirsomewhat outside of and. above its eaves.

The radialconductors- 28 may be supplement'ed, if desired, by conductingguy-wires 32, 32serving electrically-and mechanicallyto connect the ringconductor 25 at posts'27 with the marginal conductor 31. The network 30may comprise a-series of parallel wires stretched across the spaceenclosed by the'conductor 31, and not further apart-than the minimumdistance of the wires from the roof 9. The conductor- 25 may be on'thesurface of the earth or buried in a shallow trench. While any expedientmay be resorted to for supporting the network-30in the desired relationto the roof 9, a preferred expedient as shown in Figs. 3 and-4 is toprovide stout poles or metallic towers 35, 36 at either end of the majoraxis of the oval reservoir shown in these figures and to suspend fromtower to tower a cable 37 in turn supporting a transverse cable or wire38, the cable 37 having at intervals suspender wires to support the wire38 in a position substantially parallel with the ma or diameter of thereservoir roof. It will be understood that the suspenders 4O vary inlength with their'position and the catenary swept by the cable 37 Thenetwork for the reservoirs may be supported by as manytowers andsuspension cables as are necessary. Thus in Fig. 2, the support for thenetwork for the reservoir R comprises six towers designated respectively41, 42, 43, 44, and 46, and three supporting cables 47, 48 and 49, whilethe supports for the network for the reservoirs R and R comprise thetowers 35, 36 and the cables 37 previously described. It will beunderstood that the parallel wires of the net work 30 are in any caserecommended to be employed throughout the space Within the marginalconductor 31 and within the surrounding ring conductor 25. As shown thetowers supporting the suspender cables are preferably connected to'thering conductors 25, whatever the form of the construction.

In addition to the means for suspending the conducting network over theroof of the reservoir for the purpose above mentioned of providing bothan elevated conducting path and means for elevating the equipotentialplane of the earths surface above the roof of the reservoir, it isdesirable also to provide a structure to control so far as possible theplaces of culn'iinating potential of the earths surface in relation tothe reservoir or reservoirs and the conductive systems of pipes orwires. One form of such structure comprises erect receiving conductorslocated in proper relation to the region in which the reservoirs areplaced to induce lightning stroke discharge at the places of saidconductors; For example, conducting metallic towers 50, 51, 52, 53, Fig.1, may be placed in such relation to the reservoir R as to in duce theexpectation that potential rises ill culminate upon them andthat alightning stroke will be received by one or the other of these towersrather than directly by the conductive network over the reservoir or apart of the reservoir itself. These towers may be at least of-such aheight it and such a distance r from the center of the figure of thereservoir R that the assumption that the erect conductor will receivethe discharge is valid; in which case, on the assumption that a tower ofa particular height protects from stroke a circle whose radius is adefinite multiple of the height of the tower, these protected circleswill meet or overlap at the center of the reservoir. In general theplacing of the towers 50, 51, 52, is recommended to be such as toembrace an angular magnitude of the reservoir R from the tower as acenter defining an acute angle such as 717 w w, 10 of Fig. 1. ln'respectto surface flux to any of the lightning towers, the territory embracedin one of these angles defines that part of the earths surface from andto which flux to that tower would enibrace and include the reservoir, ifthe earths charge were uniformly distributed and the surfacehomogeneously conductive.

Referring now to Fig. 2, it will be ob served that when the reservoirsare crowded together as in the actual instance of practice shown in thisfigure, stroke receiving towers may be distributed according to theprinciples above explained in relation to the supply pipe network, thering conductors Q5 surrounding the reservoirs, and the remainder of theconductive network, in such a way as to utilize the service piping andthe protective network as shunt flow paths for surges to any one of thetowers 54, 55., 56, 57, 58 and 59 from the angular area or areas to thetower as a center subtended by one or more of the reservoirs.

In all cases, the lightning stroke receiving towers are recommended tobe thoroughly connected by conductors of relatively low resistance tothe piping network a and to adjacent points on the ring conductors 25.

In operation, when high potential differ ences occur between an elevatedstratum and the earth in the neighborhood of a reservoir or reservoirs,it may be assumed that fluxes of the earths charge in relation to thereservoirs and the respective lightning towers 50 to 59 occur radiallyin respect to some one of these towers, or to one or more of them insuccession. So much of this flow as is intercepted by the position of areservoir is now taken by the conductor system 25, 30; a negligibleproportion of these currents, whether principally a flow in dry weatheron the service pipe system or on wet earth aftously-close attachment ofthe reservoir. In

the case of surges of such terrestrial currents, f

the termini of conduction paths for the surges are'no longercat thereservoir roof, pipe ends or reservoir walls, but on the network 30 orone or more of the lightning towers 50 to 59. In the case of a lightningdischarge, secondary surges in the conduction system are thus shuntedaway from the gaps between conductors at or near the inflammable vapors.

The effect of the network 30, as more thoroughly explained in saidBaldwin and Schaefier patent, is moreover beneficially exercised toreduce the building up of surface charges on exterior or interiorsurfaces of the reservoir roof and to lift the equipotential surfacecorresponding to the potential of the earths surface away fromcoincidence with the reservoir roof in respect to any probability ofrupture of the dielectric by a direct stroke to the roof or walls of thereservoir instead of to the conduction network, at one of the towers 50to 59.

I claim:

1.. The combination of a structure with a series of other structures,each of the latter in the form of a lightning rod constructedindependently of the first structure, each rod spaced from the firststructure and extending to a height towering a substantial distanceabove the first structure to protect the latter against a directlightning stroke, and a metallic shield for the structure independent ofthe rods for shielding said first structure against the inductive effectof a lightning discharge and thereby preventing a discharge between twoadjacent points in the first structure.

2. Apparatus for protecting the contents of a container of inflammablefluid from atmospheric electrical ignition comprising erect earthedconductors spaced from the container a sufficient distance to insureagainst side flashes from said conductors to the container, the heightof each of said conductors bearing such relation to its distance fromthe center of figure in plan of said container, as to include saidcenter of figure in the area protected from lightning discharge bypreferential path through said erect conductor, the areas so protectedby each erect conductor overlapping each other and together covering theWhole area of said container to protectthe latter against a directlightning stroke, and a grounded conductor system independent of saiderect conductors and exterior to and surrounding the upper part at leastof the container for shielding its contents against the inductive effectof a lightning discharge, thereby to prevent a discharge betweenadjacent points in the container.

3. The combination of a structure with a series of other structureseachof the latter in the form of a lightning rod constructedindependently of the first structure, each rod spaced from the firststructure and extending to a height towering a substantial distanceabove the first structure to protect the latter against a directlightning stroke, and a metallic shield for the structure independent ofthe rods for shielding said first structure against the inductive effectof a lightning discharge and thereby preventing a discharge betweenadjacent points in the first structure, the rods being spaced from thestructure a distance sufficient to insure against side flashes from therods to the first structure.

4. The combination of a structure with a series of other structures,each of the latter in the form of a lightning rod constructedindependently of the first structure, each rod spaced from the firststructure and extending to a height towering a substantial distanceabove the first structure to protect the latter against a directlightning stroke, and a metallic shield for the structure independent ofthe rods for shielding said first structure against the inductive effectof a lightning discharge and thereby preventing a discharge betweenadjacent points in the first structure, the rods being spaced from thestructure a distance sufiicient to insure against side fiashes from therods to the first structure, said distance being not less than one halfthe height of the pole.

5. Apparatus for protecting the contents of a container of inflammablefluid from atmospheric electrical ignition comprising erect earthedtowers spaced from the container a suificient distance to insure againstside flashes from said towers to the container, the area protected byeach tower being a circle the radius of which is a definite multiple ofthe height of the tower, and said towers being so located with respectto the container that the distance to each tower from the center of thecontainer is less than 7 said radius and thereby the whole area of thecontainer is covered by said areas protected by the towers, and agrounded conductor systern independent of said towers and exterior toand surrounding said container for shielding its contents against theinductive effect of a lightning discharge, thereby to prevent adischarge between adjacent points in the container.

6. Apparatus for protecting the contents of a container of inflammablefluid from atmospheric electrical ignition comprising a ring conductorsurrounding and exterior to the container and connected in everydirection to the earth, an elevated marginal conductor connected atfrequent intervals to said ring conductor, a wire grid above and spacedfrom the roofconnected across the enclosure of said marginal conductorand lightning towers outlying in respect to said marginal conductorhaving relatively low resistance conductive conections to said ringconductor.

7 Signed by me at Boston, Massachusetts, this 14th day of January, 1927.

ELMER R. SCHAEFFER.

